Image forming apparatus

ABSTRACT

An image forming apparatus includes an apparatus main body that carries out an image forming processing with respect to a sheet. A sheet supplying unit is mounted detachably into the apparatus main body and adapted to supply a sheet from a stack of sheets stored therein toward an inside of the apparatus main body. The sheet supplying unit includes a position keeper for keeping the sheet stack in a determined position by contacting sides of the stack. The keeping position is changeable in accordance with the size of a sheet. A lock mechanism locks the position keeper in a keeping position. The locking mechanism releases the locking of the position keeper when the sheet supplying unit is detached from the apparatus main body and placed on a surface. However, the locking mechanism locks the position keeper when the sheet supplying unit is lifted from the surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus provided with a sheet supplying unit detachably mounted into an apparatus main body and adapted to supply a sheet toward an inside of the apparatus main body.

2. Description of the Related Art

An image forming apparatus usually forms an image based on image information read by a scanner or transmitted from an external equipment such as a computer, and prints the image on the sheet. The image forming apparatus includes a sheet cassette (a sheet supplying unit) detachably mounted into an apparatus main body, and the sheet cassette contains a plurality of sheets (a stack of sheets) therein. A sheet is picked up from the sheet cassette to be brought to an image forming processing.

The sheet cassette is formed with a pair of cursor members for positioning the sheet stack in a widthwise direction in accordance with a sheet size, and an end fence for positioning a rear end of the sheet stack. The sheet stack stored in the sheet cassette is positioned in a determined position in the sheet cassette by adjusting a space between inner surfaces of the pair of cursor members and a lengthwise position of the end fence. An image forming apparatus provided with such sheet cassette is disclosed in Japanese Unexamined Patent Publication No. Hei. 7-206171 and Japanese Unexamined Patent Publication No. 2000-153925, for example.

Meanwhile, the sheet cassette is formed with a lock mechanism for locking the cursor members or the end fence at a retaining position. When the cursor members or the end fence is required to be moved, an operation lever mounted in the lock mechanism is operated to release the locking, and the cursor members or the end fence is moved while keeping the operation lever to be operated.

Since it is common that the operation lever is operated in a direction orthogonal to a moving direction of the cursor members or the end fence, the cursor members or the end fence is moved in the direction orthogonal to the operating direction of the operation lever. For this reason, a difficult operation has been required. More specifically, the operation lever is pulled upward to release the locking, followed by moving the cursor members or the end fence in a lateral direction while keeping the operation lever to be pulled upward, for example. This causes a problem of an inferior operability.

SUMMARY OF THE INVENTION

In view of the above problem, it is an object of the present invention to provide an image forming apparatus which enables to release the locking of the cursor members or the end fence provided in the sheet supplying unit without operating an operation lever, thereby reducing an operator's burden.

In order to achieve the above object, an image forming apparatus according to an aspect of the present invention comprises: an apparatus main body operable to carry out an image forming processing with respect to a sheet; and a sheet supplying unit detachably mounted into the apparatus main body and adapted to supply a sheet from a stack of sheets stored therein toward an inside of the apparatus main body. The sheet supplying unit includes: a position keeper for keeping the sheet stack in a determined position by contacting with sides of the sheet stack, and the keeping position being changeable in accordance with the size of a sheet; and a lock mechanism for locking the position keeper in a keeping position, the lock mechanism releases the locking of the position keeper when the sheet supplying unit is detached from the apparatus main body and placed on an operative surface and sets the locking of the position keeper when the sheet supplying unit is lifted up from the operative surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are perspective views respectively showing a printer according to an embodiment of the present invention. FIG. 1A shows an accommodated state where the sheet cassette is accommodated in a printer main body. FIG. 1B shows a withdrawal state where the sheet cassette is substantially half withdrawn from a housing, and a rear end portion of the sheet cassette is in the printer main body. FIG. 1C shows a detached state where the sheet cassette is completely detached from the printer main body and placed on a table.

FIGS. 2A and 2B are cross sectional side views showing an outline of an inner construction of the printer. FIG. 2A shows a state where the sheet cassette is accommodated in a housing. FIG. 2B shows a state where the sheet cassette is withdrawn from the housing by substantially half.

FIGS. 3A and 3B are perspective views respectively showing a construction of the sheet cassette seen from obliquely upward. FIG. 3A shows a state where a lifting plate is mounted on the sheet cassette. FIG. 3B shows a state where the lifting plate is removed from the sheet cassette.

FIG. 4 is a perspective view showing the sheet cassette seen from a reverse side thereof.

FIG. 5 is an exploded perspective view showing a relative positional relationship of a position keeper and a lock mechanism.

FIGS. 6A to 6C are perspective views respectively showing a main part of an assembled sheet cassette. FIG. 6A is a perspective view showing a main part of the assembled sheet cassette. FIG. 6B is an enlarged perspective view showing a lock mechanism locking racks of the position keeper. FIG. 6C shows a state where the racks of the position keeper are released from the locking state of the lock mechanism.

FIGS. 7A and 7B are cross sectional views taken along the VII-VII line in FIG. 6A. FIG. 7A shows a state where a lock releasing member is set in a lower position. FIG. 7B shows a state where the lock releasing member is set in an upper position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A to 1C are perspective views respectively showing a printer according to an embodiment of the present invention. FIG. 1A shows a state where a sheet cassette is mounted in a printer main body. FIG. 1B shows a state where the sheet cassette is substantially half withdrawn from the printer main body, and a rear end portion of the sheet cassette is in the printer main body. FIG. 1C shows a state where the sheet cassette is completely detached from the printer main body, and placed on a table. In FIGS. 1A to 1C, it should be noted that X-X direction and Y-Y direction are referred to as a widthwise direction and a forward and backward direction, respectively, wherein a −X direction is a leftward direction, a +X direction is a rightward direction, a −Y direction is a forward direction and a +Y direction is a backward direction.

A printer (image forming apparatus) 10 is adapted to conduct an image forming processing in accordance with an image information inputted from an external device such as a computer and the like, and to conduct a transferring of an image on a sheet P. The printer 10 is constructed by accommodating units necessary for forming an image in a housing 11 having a box shape whose top portion is in an upwardly curved shape in a side view.

The housing 11 includes a pair of side plates 111 in the widthwise direction, a top plate 112, a front plate 113, a rear plate 114 (FIG. 2A), and a partition plate 115 (FIG. 2A). Each of the pair of side plates 111 has an arc top edge line. The top plate 112 is arranged on the top ends of the pair of the side plates 111, and has an upwardly curved shape in a side view. The front plate 113 extends from a front end of the top plate 112 to a substantially middle position in a vertical direction. The rear plate 114 extends from a rear end of the top plate 112 to a substantially middle position in the vertical direction. The partition plate 115 is bridged between the pair of the side plates 111 at the substantially middle position in the vertical direction.

As shown in FIG. 1C, the housing 11 has no bottom plate, and respective bottom ends of the pair of right and left side plates 111 and the rear plate 114 are directly placed on a flat table surface. Further, the housing 11 includes above the partition plate 115 an image forming section 20 (FIG. 2A) having later described units necessary for forming an image. Below the partition 115, a sheet cassette (sheet supplying device) 30 described later is detachably mounted.

A sheet supplying opening 116 is formed below a lower edge of the front plate 113. A manual sheet supplying tray 35 is supported by the sheet cassette 30, and is positioned immediately below the lower edge of the front plate 113. When a sheet P is supplied manually, the sheet P is introduced in the housing 11 through the sheet supplying opening 116 with the manual sheet supplying tray 35 being withdrawn frontward from the sheet cassette 30.

The top plate 112 is formed with a discharging tray 117 by slopping down a center portion of the top plate toward the rear end thereof to form a concave space. The sheet P on which a predetermined image forming processing is conducted in the housing 11 is discharged on the discharging tray 117 through a discharging opening 117 a opened in a rear wall of the discharging tray 117.

Further, each of the side plates 111 is formed with a louver 118 in a rear upper portion thereof. The louver 118 is provided with a number of gratings extending in the vertical direction and arranged in parallel in the forward and backward direction. One of the louvers 118 takes external air and the other one exhausts it to thereby cool the inner space of the housing 11.

The sheet cassette 30 is shifted from an accommodated state S1 that the sheet cassette 30 is accommodated in the housing 11 to operatively respond to the image forming processing of the printer 10, as shown in FIG. 1A, to a withdrawal state S2 that the sheet cassette 30 is withdrawn from the housing 11 in order to replenish sheets P, as shown in FIG. 1B. Meanwhile, when the size of sheets P to be supplied in the sheet cassette 30 is required to change, the sheet cassette 30 is completely detached from the housing 11, that is a detached state S3, and placed on a table T (a flat surface) within reach, as shown in FIG. 1C.

FIGS. 2A and 2B are cross sectional side views illustrating an outline of an inner construction of the printer 10. FIG. 2A shows a state where the sheet cassette 30 is accommodated in the housing 11 (the accommodated state S1). FIG. 2B shows a state where the sheet cassette 30 is withdrawn from the housing 11 by substantially half (the withdrawal state S2). It should be noted that directions indicated by Y (forward and backward direction) in FIGS. 2A and 2B are identical to those in FIGS. 1A to 1C (−Y: forward direction, +Y: backward direction).

As shown in FIGS. 2A and 2B, the printer 10 basically includes the image forming section 20, a fixing section 27, and the sheet cassette 30. The image forming section 20 conducts forming and transferring of an image on a sheet P. The fixing section 27 conducts fixing of the image transferred on the sheet P. The sheet cassette 30 is adapted to store a stack of sheets P1 and to supply the sheets P one by one from the stored sheet stack P1 toward the image forming section 20.

The image forming section 20 conducts forming and transferring of image on the sheet P in accordance with image information transmitted from a computer and the like. The image forming section 20 includes a photoconductive drum 21, a charging device 22, an exposing device 23, a developing device 24, a transferring roller 25, and a cleaning device 26 in a clockwise direction from an immediately above of the photoconductive drum 21 to be arranged along a periphery of the photoconductive drum 21 mounted rotatably about a drum shaft extending in the widthwise direction (a direction orthogonal to a sheet surface of FIG. 2).

The photoconductive drum 21 is adapted to form an electrostatic latent image and a toner image of the electrostatic latent image on the peripheral surface of the photoconductive drum 21. The photoconductive drum 21 has a strong and very smooth amorphous silicon layer on the peripheral surface thereof to thereby assure the formation of the electrostatic latent image or the toner image.

The charging unit 22 is adapted to uniformly charge over the peripheral surface of the photoconductive drum 21 rotating about the drum shaft in the clockwise direction. In the present embodiment, the charging unit 22 is operated in the corona discharge manner which gives electrical charge onto the peripheral surface of the photoconductive drum 21 by a corona discharge from a wire. In stead of the corona discharge manner, the charging roller manner may be adopted in which a charging roller charges over a peripheral surface of a photoconductive drum rotating with a rotation of the photoconductive drum 21 owing to contact with the photoconductive drum 21.

The exposing device 23 is adapted to illuminate the peripheral surface of the rotating photoconductive drum 21 by a laser beam having a variety of light intensities in accordance with the image information transmitted from the external equipment such as a computer and the like. An electrostatic latent image is formed on the peripheral surface of the photoconductive drum 21 by removing the electrical charge at specified portions of the charged peripheral surface of the photoconductive drum 21 by the illumination of the laser beam.

The developing device 24 is adapted to form a toner image on the peripheral surface of the photoconductive drum 21 by supplying toner particles of a developing agent to the peripheral surface of the photoconductive drum to adhere the portion where the static latent image is formed. The developing agent is a so-called two-component type agent which includes toner particles and carrier particles. The toner particles are produced by dispersing a coloring agent, an electrical charge controlling agent, a wax, and other additives in a binder resin, and shaping particles having a diameter of 6 to 12 μm. The carrier particles include magnetic particles made of a magnetic mine (Fe₃O₄) and the like and having a diameter of 60 to 200 μm, and are used to charge toner particles.

The transferring roller 25 is adapted to transfer the positively charged toner image formed on the peripheral surface of the photoconductive drum 21 onto the sheet P fed immediately below the photoconductive drum 21. The transferring roller 25 provides the sheet P with negative electrical charge which is the opposite polarity to the toner image.

The sheet P having passed immediately below the photoconductive drum 21 is pressedly held by the transferring roller 25 and the photoconductive drum 21 to transfer the positively charged toner image on the peripheral surface of the photoconductive drum 21 onto the negatively charged surface of the sheet P. Thus, the transferring processing is conducted on the sheet P.

The cleaning device 26 is adapted to clean the peripheral surface of the photoconductive drum 21 by removing toner particles remaining on the peripheral surface of the photoconductive drum 21 after completing the transferring of the image onto the sheet P. The peripheral surface of the photoconductive drum 21 cleaned by the cleaning device returns to the charging device 22 again for next image forming processing.

The fixing section 27 is adapted to conduct fixing the transferred toner image on the sheet P by heat. The fixing section 27 includes a heat roller 271 having a heating element such as a halogen lump and the like, and a pressing roller 272 which is arranged below the heat roller 271 and whose peripheral surface faces a peripheral surface of the heat roller 271. The sheet P after the transferring processing comes into a nip portion between-the heat roller 271 drivingly rotating in the clockwise direction about a roller shaft and the pressing roller 272 drivenly rotating in the counterclockwise direction about the roller shaft, and receives heat from the heat roller 271, so that the image is fixed on the sheet P. The sheet P having subjected to the fixing processing is discharged to the discharge tray 117 through a discharging passage 273.

The housing 11 is provided with a sheet conveying roller 119 having a large diameter on an upstream of the sheet cassette 30 (the front side in the FIG. 2A). The sheet conveying roller 119 is adapted to pick up sheets P one by one from the sheet stack P1. Immediately above the sheet supplying roller, a conveying roller 119 a having a small diameter is arranged. Meanwhile, the sheet cassette 30 includes a sheet roller 301 facing the conveying roller 119 a. The combination of conveying roller 119 a and the sheet roller 301 is served as a pair of sheet supplying rollers. The sheet P picked up from the sheet cassette 30 by the driving of the sheet conveying roller 119 is forwarded to the image forming section 20 through the rollers 119 a and 301, a sheet conveying passage 119 b, and a pair of registration rollers 119 c provided on a downstream of the sheet conveying passage 119 b.

Hereinafter, the sheet cassette 30 is described in more detail referring to FIGS. 3 to 5. FIGS. 3A and 3B are perspective views respectively showing a construction of the sheet cassette 30 seen from obliquely upward. FIG. 3A shows a state where a lifting plate 36 is mounted on the sheet cassette 30. FIG. 3B shows a state where the lifting plate 36 is removed from the sheet cassette 30. FIG. 4 is a perspective view showing the sheet cassette 30 seen from a reverse side thereof. Further, FIG. 5 is an exploded perspective view illustrating a relative positional relationship of a cursor member 42 and a lock mechanism 50.

Further, FIGS. 6A to 6C are perspective views respectively showing a main part of the assembled sheet cassette 30. FIG. 6A is a perspective view of the main part of the assembled sheet cassette 30. FIG. 6B is an enlarged perspective view showing a lock mechanism 50 locking racks 424 provided on the pair of cursor members 42. FIG. 6C shows a state where the racks 424 of the pair of cursor members 42 are released from the locking state of the lock mechanism 50.

It should be noted that directions indicated by X and Y in FIGS. 3 to 6 are identical to those in FIG. 1 (X is widthwise direction (−X: leftward direction, +X: rightward direction), Y is forward and backward direction (−Y: forward direction, +Y: backward direction).

As shown in FIGS. 3A and 3B, the sheet cassette 30 includes a cassette main body 31, a position keeper 40, and a lock mechanism 50 (FIGS. 3B, 4, and 5). The cassette main body 31 has a shallow square dish-like shape. The position keeper 40 is provided inside the cassette main body 31, and adapted to keep the sheet stack P1 in a determined position. The lock mechanism 50 is adapted to set the locking of the position keeper 40, and release the locking of the position keeper 40.

The cassette main body 31 includes a bottom plate 32, a pair of side plates 33, and a decorative plate 34. The bottom plate 32 has a rectangular shape. The pair of side plates 33 rise from opposite side ends of the bottom plate 32, and extend in the frontward and backward direction. The decorative plate 34 has a longer size in a vertical direction than the side plates 33 rising from the front end of the bottom plate 32. In a back side of the decorative plate 34, a manual sheet supplying tray 35 is mounted for supplying a sheet P to the image forming section 20 manually. When the sheet P is supplied by hand, the manual sheet supplying tray 35 is pulled out forward from the decorative plate 34. A storing space V for storing the sheet stack P1 is assured by being surrounded with the side plates 33 on the bottom plate 32 and the manual sheet supplying tray 3.

The cassette main body 31 is detachably mounted on the housing 11 along a pair of guiding rails 331 in the vertical direction (FIG. 2B) which are respectively provided on the lower portions of the opposite surfaces of the side plates 33 of the housing 11, and extend in the forward and backward direction. The bottom plate 32 is supported by these guide rails 331 to form a space having a predetermined size between an upper surface of a pedestal T1 on which the printer 10 is placed and an underside surface of the bottom plate 32. The space allows a later described lock releasing member 52 to be positioned in a lower position S4 where the lock releasing member 52 projects downwardly from the bottom plate 32 as shown in FIG. 2A while the sheet cassette 30 is accommodated in the housing 11.

In the storing space V, the lifting plate 36 is provided to lift an upstream end (front end) of the sheet stack P1 loaded in the storing space V as shown in FIG. 3A. The lifting plate 36 has a length in the forward and backward direction from a substantially middle portion of the bottom plate 32 to the manual sheet supplying tray 35 in the frontward and backward direction. The lifting plate 36 includes supporting shafts 361 respectively projecting in the opposite directions from each other in transversely opposite ends of backward end of the lifting plate 36. These supporting shafts 361 are supported by the side plates 33 in an axially rotatable manner so that the lifting plate 36 rotates forwardly and reversely about the supporting shafts 361. Further, the lifting plate 36 is formed with a pair of recesses 362 in the opposite side ends of the lifting plate 36. These recesses 362 provide a space to ensure movement of the later described pair of cursor members 42.

The bottom plate 32 is provided with a pair of coil springs 321 in the widthwise direction in a portion opposite to a front portion of the lifting plate 36 (a later described second bottom plate 32′) as shown in FIG. 3B. The pair of coil springs 321 urges the front portion of the lifting plate 36 upward. Thus, the sheet cassette 30 is inserted into the housing 11 in the state that the sheet stack P1 is loaded on the lifting plate 36, so that an upper surface of the front end of the sheet stack P1 comes into contact with a peripheral surface of the sheet conveying roller 119 (FIG. 2A), with the result that a front end of the lifting plate 36 moves downward against the biasing force of the coil springs 321 through the sheet stack P1. When the sheet cassette 30 is completely accommodated in the housing 11, an uppermost sheet P of the sheet stack P1 pressedly contacts to the peripheral surface of the sheet conveying roller 119. Thus, the uppermost sheet P in the sheet stack P1 is picked up toward the rollers 119 a and 301 by the rotation of the sheet conveying roller 119 in the counterclockwise direction about the shaft as shown in FIG. 2A.

The position keeper 40 includes a rear end positioning cursor member 41 and the pair of cursor members 42. The rear end positioning cursor member 41 keeps a downstream end (a backward end) position of the sheet stack P on the lifting plate 36 in the storing space V. The pair of cursor members 42 keeps the sheet stack P1 in a center in the widthwise direction. The rear end positioning cursor member 41 is movable forward and backward directions along a pair of guide rails 322 which are provided on a backward portion of the bottom plate 32 and extend in the forward and backward direction. Further, the rear end positioning cursor member 41 includes a retaining mechanism. The rear end positioning cursor member 41 is retained at a fixed position by operating the retaining mechanism in a predetermined direction, and released by operating the retaining mechanism in the reverse direction.

Hereinafter, the cursor members 42 and the lock mechanism 50 are described referring to FIGS. 5, 6A, 6B, and FIGS. 1 to 4 according to the necessity.

As shown in FIG. 5, the cursor members 42 are mounted on the second bottom plate 32′ placed over a substantially front half of the bottom plate 32. The second bottom plate 32′ includes a guide hole 323 for guiding the movement of the cursor members 42 and extending in the widthwise direction. The guide hole 323 includes a forward guide hole 324 and a backward guide hole 325 shifted off from a forward guide hole 324 in the widthwise direction. The forward guide hole 324 extends in the rightward direction from a proximity of a left end of the second bottom plate 32′ to a position slightly exceeding a center portion in the widthwise direction. The backward guide hole 325 extends to the leftward direction from a proximity of the right end of the second bottom plate 32′ to a position slightly exceeding the center portion in the widthwise direction. These forward and backward guide holes 324 and 325 have an overlapped portion capable of communicating with each other.

There are provided a left cursor member 421 and a right cursor member 422. The left cursor member 421 is moved along the forward guide hole 324. The right cursor member 422 is moved along the backward guide hole 325. Each of the cursor members 421 and 422 includes a cursor plate 423 extending in the forward and backward direction, and a rack 424 extending in the widthwise direction. The racks 424 extend from respective bottoms of the cursor plates 423 toward the center. The rack 424 of the left cursor member 421 is inserted in the forward guide hole 324 in a state where a left end portion of the rack 424 is fixedly attached to a bottom end of the left cursor member 421. Meanwhile, the rack 424 of the right cursor member 422 is inserted in the backward guide hole 325 in a state where a right end portion is fixedly attached to a bottom end of the right cursor member 422. Either of the cursor members 421 or 422 can move in the widthwise directions along the guide hole 323. The racks 424 are formed with driving teeth 424 a in respective inner surfaces facing each other.

The guide hole 323 includes in a central portion in the widthwise direction a bridge plate 326 crossing the guide hole 323 in the forward and backward direction in a reverse side of the second bottom plate 32′. The bridge plate 326 includes a threaded hole 326 a which fixedly engages with a lower end thread portion of a center shaft 426 penetrating through a center hole 425 a of a pinion 425. While the center shaft 426 penetrates through the center hole 425 a, the lower end thread portion of the center shaft 426 is fixedly screwed in the center hole 425 a to axially support the pinion 425 about the center shaft 426 to allow the pinion 425 to rotate freely. The pinion 425 has such a diameter that the pinion 425 engages with the pair of racks 424 opposing to each other.

Accordingly, when one of the cursor plates 423 moves in the widthwise direction, the movement is transmitted via the rack 424 fixedly attached to the moved cursor plate 423 and the pinion 425 to the other rack 424 fixedly attached with the other cursor plate 423, thereby rendering the other cursor plate 423 move by the same amount as the moved cursor plate in the opposite direction.

Further, the rack 424 fixedly attached to the right cursor member 422 is formed with a number of round-shaped teeth (recess and projection portion) 424 b at an equal pitch in an outer surface thereof over the substantially whole length in the widthwise direction. The round-shaped teeth 424 b includes a projecting portion projecting from the rack 424 and having an arc shape of substantially half circle, as shown in FIG. 6B. The round shaped teeth 424 b are operable to lock the rack 424 with the lock mechanism 50.

Meanwhile, the second bottom plate 32′ includes a supporting portion 327 (FIG. 5) formed in such a manner that the second bottom plate 32′ is cut in a rightward portion thereof from the guide hole 323 backwardly. The supporting portion 327 is adapted to mount the lock mechanism 50. The supporting portion 327 includes guide grooves 327 a extending in the forward and backward direction in respective surfaces facing each other in the widthwise direction.

Further, the bottom plate 32 is formed with a rectangular hole 328 in a portion corresponding to the bridge plate 326 to receive the bridge plate 326. The bottom plate 32 is further formed with a recess 329 having a rectangular shape in a plan view in a portion corresponding to the supporting portion 327. The rectangular hole 328 is adapted to fit the bridge plate 326 on the second bottom plate 32′ which is in the state of being placed on and fixedly connected to the bottom plate 32 by screws. The recess 329 is adapted to ensure a vertical movement of a later described lock releasing member 52. In a center of the recess 329, a rectangular hole 329 a is formed to allow a later described rectangular column portion 521 to penetrate therethrough.

As shown in FIGS. 5 and 6B, the lock mechanism 50 includes a lock plate 51, the lock releasing member 52, and a coil spring 53. The lock plate 51 is movably engaged with the supporting portion 327 in the forward and backward direction. The lock releasing member 52 is operable to move the lock plate 51 in the backward direction in the state where the sheet cassette 30 is detached from the housing 11 and placed on the table T as shown in FIG. 1C. The coil spring 53 urges the lock plate 51 in the supporting portion 327 in the forward direction. The coil spring 53 is mounted in the compressed state between the lock plate 51 and the backward end portion of the supporting portion 327.

The lock plate 51 is adapted to lock the cursor member 42, and formed in a substantially square shape. The lock plate 51 includes guide projections 511 extending in the forward and backward direction in the leftward and rightward sides of the lock plate 51. The lock plate 51 is further formed with a rectangular hole 512 for passing through a later described actuating portion 523 in a center of the lock plate 51. The lock plate 51 is formed with a plurality of sawteeth (locking teeth) 513 in a forward surface thereof facing the plurality of round shaped teeth 424 b formed in the rack 424.

The pair of guide projections 511 are engaged with the pair of the guide grooves 327 a of the supporting portion 327. Accordingly, each of the guide projections 511 is engaged with the corresponding guide groove 327 a so that the lock plate 51 is movable in the forward and backward direction in the supporting portion 327 while being guided by each of the guide grooves 327 a.

Each of the sawteeth 513 has a leading edge formed sharply (FIG. 6C). The plurality of sawteeth 513 are formed at the same pitch as the round shaped teeth 424 b. Accordingly, when the lock plate 51 moves forward in the supporting portion 327, the leading edges of the sawteeth 513 are engaged with recesses of the round shaped teeth 424 b. This arrangement prevents the movement of the rack 424.

The rectangular column portion 521 penetrates the rectangular hole 329 a (FIGS. 4 and 5) formed in the recess 329 of the bottom plate 32. The lock releasing member 52 is further formed with a flange portion 522 integrally formed with the rectangular column portion 521 in an upper end of the rectangular column portion 521. The actuating portion 523 projects upward from an upper surface of the flange portion 522. The rectangular hole 329 a formed in the recess 329 has substantially the same size as the rectangular hole 512 formed in the lock plate 51 in the plan view.

The flange portion 522 has a size larger than the rectangular hole 329 a of the recess 329, thereby preventing the rectangular column portion 521 from falling off when the rectangular column portion 521 is inserted in the rectangular hole 329 a from upward. The actuating portion 523 has a section having a right triangle shape from the side view (in the widthwise direction) as shown in FIGS. 6B and 6C to thereby define an inclined surface 523 a facing backward.

When the lock releasing member 52 is set in the lower portion S4 (FIG. 2A), as shown in FIG. 6B, the actuating portion 523 is at a lowest position in the rectangular hole 512 by the weight of the lock releasing member 52. Thus, the lock plate 51 moves forward by the biasing force of the coil spring 53. A backward end of the rectangular hole 512 contacts with an upper end of the actuating portion 523 to allow the sawteeth 513 to engage with the round shaped teeth 424 b of the rack 424.

On the other hand, when the sheet cassette 30 is detached from the housing 11 and placed on the table T to cause the rectangular column portion 521 to receive the reaction force from the pedestal T1 or the table T, the lock releasing member 52 moves upward against the biasing force of the coil spring 53. With this arrangement, the inclined surface 523 a of the actuating portion 523 presses the backward end of the rectangular hole 512 of the lock plate 51 in the backward direction so that the lock plate 51 moves in the backward direction, thereby releasing the engagement of the sawteeth 513 with the round shaped teeth 424 b as shown in FIG. 6C.

FIGS. 7A and 7B are cross sectional views taken along the line VII-VII in FIG. 6A, showing an operation of the lock mechanism 50. FIG. 7A shows a state where the lock releasing member 52 is set in the lower position S4. FIG. 7B shows a state where the lock releasing member 52 is set in an upper position S5. It should be noted that indication of directions by X and Y in FIGS. 7A and 7B are the same as those in the FIG. 1 (X indicates the leftward and rightward direction (−X: leftward, +X: rightward), Y indicates the forward and backward direction (−Y: forward, +Y: backward).

First, as shown in FIG. 7A, the lock releasing member 52 is set in the lower position S4 by the weight when: 1) the sheet cassette 30 is accommodated in the housing 11, and 2) the sheet cassette 30 is detached from the housing 11 and then placed on the table T. At this time, only an upper end of the actuating portion 523 comes in the rectangular hole 512 of the lock plate 51 until the lock plate 51 moves forward and the backward end of the rectangular hole 512 contact with the upper end of the actuating portion 523 due to the biasing force by the coil spring 53.

Under this condition, the sawteeth 513 of the lock plate 51 engages with the round shaped teeth 424 b of the rack 424 to thereby lock the movement of the rack 424 of the right cursor member 422 in the widthwise direction (in the direction orthogonal to the sheet surface of FIG. 7A). Accordingly, the cursor members 42 set in the determined position are kept from moving when the sheet cassette 30 is accommodated in the housing 11 or withdrawn from the housing 11 as shown in FIGS. 2A and 2B, or when the sheet cassette 30 detached from the housing 11 and then placed on the table T, or when the sheet cassette 30 is lifted up from the table T and then accommodated in the housing 11.

Accordingly, the cursor members 42 once set in the determined position are kept from moving even due to a vibration when accommodating or detaching the sheet cassette 30 in or from the housing 11, or an accidental force caused when the sheet cassette 30 is carried. Accordingly, the keeping position of the cursor member 42 is assuredly prevented from going wrong while the sheet cassette 30 is accommodated in the housing 11.

Next, as shown in FIG. 7B, when the sheet cassette 30 detached from the housing 11 is placed on the table T, the rectangular column portion 521 of the lock releasing member 52 is pushed up by a reaction force from the table T. Consequently, the rock releasing member 52 moves upward against the biasing force by the coil spring 53, and the inclined surface 523 a of the actuating portion 523 pushes the backward end of the rectangular hole 512 of the lock plate 51, thereby moving the lock plate 51 backward to release the engagement of the sawteeth 513 with the round teeth 424 b, and permit the cursor members 42 to move.

Accordingly, when the cursor members 42 are required to be moved in order to follow a size of a changed sheet stack P1 stored in the sheet cassette 30, the sheet cassette 30 is detached from the housing 11 and placed on the determined table T. In this way, the cursor members 42 are permitted to be moved without the necessity of actuating a special operating lever as the conventional machines. Therefore, the workability at the size change of sheets can be improved.

As described above, the printer 10 according to the present embodiment includes the sheet cassette 30 capable of supplying a sheet P from the stored sheet stack P1 to the inside of the housing 11 for the image formation, and detachably mounted in the housing 11. The sheet cassette 30 includes the position keeper 40 and the lock mechanism 50. The position keeper 40 is capable of moving in accordance with the size of sheets P to contact with the sides of the sheet stack P1, thereby keeping the sheet stack P1 in a determined position. The lock mechanism 50 sets and releases the locking of the position keeper 40. The lock mechanism 50 releases the locking when the sheet supplying cassette 30 is detached form the housing 11 and placed on the table T, while the lock mechanism 50 sets the locking when the sheet cassette 30 is lifted up from the table T.

With this arrangement, the sheet stack P1 is kept in a determined position in accordance with a sheet size by the position keeper 40 and the position keeper 40 is locked in the keeping position by the locking mechanism 50. Thus, the stack of sheets are stored in the sheet cassette 30 stably.

When a sheet stack P1 having a different size is loaded in the sheet cassette 30, the locking of the lock mechanism 50 can be released by detaching the sheet cassette 30 from the housing 11 and placing it on the predetermined table T. Under this condition, an operation to set the position keeper in accordance with the new sheet size can be carried out by moving the position keeper 40. The sheet cassette 30 storing the sheet stack P1 having the new sheet size is accommodated in the housing 11.

In the present embodiment, since the state where the sheet cassette 30 is accommodated in the housing 11 is the same situation as the state where the sheet cassette 30 is lifted up from the table T, the position keeper 40 is locked in the keeping position by the lock mechanism 50 as far as the housing 11 does not have any operative member coming into contact with the lock leasing member 52.

The lock mechanism 50 releases the locking of the position keeper 40 when the sheet cassette 30 is detached from the housing 11 and placed on the predetermined table T, while setting the locking of the position keeper 40 when the sheet cassette is lifted up from the table T. With this arrangement, an operator is not required to operate a special operative lever for releasing the locking to move a position keeper as the conventional machines. The operator can move the position keeper 40 only by placing the sheet cassette 30 detached from the housing 11 on the table T, thereby facilitating the task to change the position of the position keeper 40 in accordance with the sheet size, with the result that the workability of this task is remarkably improved.

Further, the rack 424 which is a component of the position keeper 40 includes the round-shape teeth 424 b having the plurality of recesses and projections arranged at a constant pitch in the moving direction. The lock mechanism 50 includes the lock plate 51 and the actuating portion 523. The lock plate 51 includes the sawteeth 513 for locking the movement of the position keeper 40 by being engaged with the recesses and projections of the round shaped teeth 424 b. The actuating portion 523 is operable to move the rock plate 51 in the direction that the sawteeth 513 disengages from the round-shaped teeth 424 b due to the pushing force from the table T in the state where the sheet cassette 30 is placed on the table T. With this arrangement, while the sheet cassette 30 is accommodated in the housing 11, the sawteeth 513 of the lock plate 51 engages with the recesses and the projections of the round-shaped teeth 424 b so that the position keeper 40 is locked in the determined position.

When the position keeper 40 is required to move, the sheet cassette 30 is detached from the housing 11 and placed on the table T. This causes the lock releasing member 52 to receive the pushing force from the table T so that the lock plate 51 moves to release the sawteeth 513 formed in the lock plate 51 from the engagement with the recesses and projections of the round-shape teeth 424 b of the position keeper 40. Accordingly, the position keeper 40 is permitted to be moved.

As described above, the lock mechanism 50 includes the lock plate 51 having the sawteeth 513, and the lock releasing member 52 having the actuating portion 523. Thus, the construction of the lock mechanism 50 can be simplified, and the position keeper 40 is to be movable only when the sheet cassette 30 is placed on the predetermined table T. Accordingly, the production costs of the sheet cassette 30 including the lock mechanism 50 can be suppressed.

Further, the actuating portion 523 is formed with the inclined surface 523 a in the position of contacting the lock plate 51. The inclined surface 523 a has an inclination set to lead the lock plate 51 to release the locking as the lock releasing member 52 is pushed upward. Accordingly, when the sheet cassette 30 is placed on the predetermined table T, the lock releasing member 52 is relatively pushed upward to cause the inclined surface 523 a to lead the lock plate 51. Thus, the lock plate 51 moves in the direction of releasing the locking so that the position keeper 40 can be released from the locking state.

Further, in the present embodiment, the position keeper 40 includes the pair of cursor members 42 each having the lock mechanism 50 in the widthwise direction. The pair of the cursor members 42 hold the sheet stack P1 from the opposite sides by being associated with each other. Accordingly, the operability of the cursor members 42 whose operation is more difficult than that of the rear end positioning cursor member 41 is improved. Additionally, the pair of the cursor members 42 can keep the sheet stack P1 stored in the sheet cassette 30 in the determined widthwise direction.

The present invention is not limited to the above embodiments, but may be modified as follows.

(1) In the above embodiments, the image forming apparatus is described as the printer 10. However, according to the present invention, the image forming apparatus is not limited to a printer, but may be a copier, a facsimile machine, or the like.

(2) In the above embodiments, the lock mechanism 50 is provided to operate the cursor members 42. However, according to the present invention, the lock mechanism is not limited to the operation of only the cursor members 42. In addition to the above, the lock mechanism may be made possible to operate the rear end positioning cursor member 41. Further, the lock mechanism 50 may be provided to operate only the rear end positioning cursor member 41 without operating the cursor member 42.

(3) In the above embodiment, the sheet cassette 30 is accommodated in the housing 11 only at a single level. However, the present invention is not limited to the accommodation of a sheet cassette at the single level. The housing 11 may be formed to be capable of accommodating a plurality of sheet cassettes at a plurality of levels.

(4) In the above embodiment, the rack 424 of the rightward cursor member 422 includes the round-shape teeth 424 b for the locking. However, the present invention is not limited to the above. Gear teeth usually used or sawteeth having the sharp leading edges may be adopted.

(5) In the above embodiment, the position keeper 40 includes the pair of cursor members 421, 422. However, a single cursor member may be adopted in the case where a sheet cassette is so constructed that sheet stack P1 is kept in the predetermined position by being approached at one side in the sheet widthwise direction.

(6) In the present invention, the operating member 523 of the lock releasing member 52 includes the inclined surface 523 a. However, instead thereof, the rectangular hole 512 of the lock plate 51 may be defined by an inclined surface in the end portion thereof, or both of the rectangular hole and the operating member may be formed with an inclined surface.

As described above, an inventive image forming apparatus comprises an apparatus main body operable to carry out an image forming processing with respect to a sheet; and a sheet supplying unit detachably mounted into the apparatus main body and adapted to supply a sheet from a stack of sheets stored therein toward an inside of the apparatus main body. The sheet supplying unit includes a position keeper for keeping the sheet stack in a determined position by contacting with sides of the sheet stack, the keeping position being changeable in accordance with the size of a sheet; a lock mechanism for locking the position keeper in a keeping position. The lock mechanism releases the locking of the position keeper when the sheet supplying unit is detached from the apparatus main body and placed on a flat table surface and setting the locking of the position keeper when the sheet supplying unit is lifted up from the operative surface.

With the above arrangement, the sheet stack is loaded in the sheet supplying unit along the position keeper previously set in the predetermined position in accordance with a sheet size and locked by the locking mechanism in the keeping position, therefore the sheet stack can be stored in the sheet cassette in the keeping position.

When a stack of sheets having a different size is required to be loaded in the sheet supplying unit, the locking by the lock mechanism can be released by placing the sheet supplying unit on a flat table surface after detaching the sheet supplying unit from the apparatus main body. Accordingly, the position keeper can be changed to a different sheet size under this condition. The sheet supplying unit storing the stack of sheets having the different sheet size is mounted in the apparatus main body.

The state where the sheet supplying unit is accommodated in the apparatus main body is in the same condition as the state where the sheet supplying unit is lifted up from the flat table surface. Accordingly, when being placed in the apparatus main body, the sheet supplying unit is unchangedly kept in the state caused when the sheet supplying unit is lifted up from the flat table surface, that is, the state that the position keeper is locked in the keeping position by the lock mechanism.

The position keeper releases the keeping position when the sheet supplying unit is detached from the apparatus main body and placed on the flat table surface, while the locking is set when the sheet supplying unit is lifted up from the flat table surface. With this arrangement, an operator is not required to operate an operation lever for releasing the locking and move the position keeper, as in the conventional manner. The position keeper can be freely moved only by placing the sheet supplying unit on the flat table surface. Therefore, the position change of the position keeper according to the sheet size is extremely facilitated, with the result that the workability of this task is remarkably improved.

Further, it is preferable that the position keeper includes a recess and projection portion having a plurality of recesses and projections alternately formed at the predetermined pitch in the moving direction thereof, and the lock mechanism includes a lock plate having locking teeth engageable with the recess and projection portion of the position keeper to keep the position keeper in the keeping position, and an operating member operable to move the lock plate in the direction of disengaging the locking teeth from the recess an projection portion of the position keeper by receiving a pushing force from the operative surface when the sheet supplying unit is placed on the operative surface.

With this arrangement, in the state where the sheet supplying unit is accommodated in the apparatus main body, the locking teeth are engaged with the recesses and projections of the recess and projection portion formed on the position keeper. Thus, the position keeper is locked in the keeping position.

When the position keeper is required to be moved, the sheet supplying unit is detached from the apparatus main body and placed on the flat table surface. Thus, the operating member receives the pushing force from the flat table surface so that the lock plate moves and the locking teeth formed in the lock plate is disengaged from the recesses and projections of the recess and projection portion of the position keeper. Accordingly, the position keeper is to be movable.

The lock mechanism includes the lock plate having the locking teeth and the operating member. Accordingly, the construction of the lock mechanism can be simplified, and the position keeper is movable only when the sheet supplying unit is placed on the flat table surface. Thus, the costs incurred for the apparatus can be suppressed.

It is preferable that one or both of the lock plate and the operating member have the inclined surface in the contact portion where the lock plate and the operating member contact each other, and the inclined surface has the inclination such that the operating member moves the lock plate to release the locking of the position keeper when the operating member receives the pushing force.

With this arrangement, the inclined surface provided in the contact portion of one or both of the lock plate and the operating member leads the lock plate to move in the direction of releasing the locking. The sheet supplying unit is placed on the flat table surface to thereby cause the operating member to receive the pushing force. Accordingly, the position keeper is released from the keeping state.

It is preferable that the position keeper includes a pair of cursor members arranged in the sheet widthwise direction and operatively connected with each other, and the pair of cursor members is operable to hold the sheet stack on the opposite sides of the sheet stack.

This arrangement can improve the operability of the cursor members, which is more difficult than the operability of the rear end positioning cursor member usually provided in the sheet supplying unit. The cursor members can adjust an entirety of the sheet stack stored in the sheet supplying unit in the sheet widthwise direction.

Moreover, it is preferable that the pair of cursor members is operable with each other by a mechanism using a rack and a pinion. Each of the cursor members is attached with a long rack extending form the bottom of the cursor member and formed with driving teeth in the side surface facing the pinion. At least one of racks is formed with a recess and projection portion having a plurality of recesses and projections at the predetermined pitch on the side surface opposite to the surface where the driving teeth are formed. The lock mechanism includes a lock plate having locking teeth engageable with the recesses and the projections of the recess and projection portion of the position keeper to keep the position keeper in the keeping position. The operating member moves the lock plate in the direction of disengaging the locking teeth from the recess and projection portion of the position keeper by receiving a pushing force from the flat table surface when the sheet supplying unit is placed on the flat table surface. With this arrangement, the lock mechanism can be operated by a part of a rack which is generally used in conventional machines.

It is preferable that the projections of the recess and projection portion each have a round-shape, and the locking teeth each have a sharp leading edge. With this arrangement, the engagement of the recess and projection portion with the locking teeth can be smoothly carried out.

This application is based on Japanese Patent Application No. 2006-010686 filed on Jan. 19, 2006, the contents of which are hereby incorporated by reference.

Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein. 

1. An image forming apparatus comprising: an apparatus main body operable to carry out an image forming processing with respect to a sheet; and a sheet supplying unit detachably mounted into the apparatus main body and adapted to supply a sheet from a stack of sheets stored therein toward an inside of the apparatus main body, wherein: the sheet supplying unit includes: a position keeper for keeping the sheet stack in a determined position by contacting with sides of the sheet stack, and the keeping position being changeable in accordance with the size of a sheet; and, a lock mechanism for locking the position keeper in a keeping position, the lock mechanism releases the locking of the position keeper when the sheet supplying unit is detached from the apparatus main body and placed on an operative surface and sets the locking of the position keeper when the sheet supplying unit is lifted up from the operative surface.
 2. An image forming apparatus according to claim 1, wherein: the position keeper includes a recess and projection portion having a plurality of recesses and projections alternately formed at a predetermined pitch in a moving direction thereof, the lock mechanism includes: a lock plate having a locking teeth engageable with the recess and projection portion of the position keeper to keep the position keeper in the keeping position; and, an operating member operable to move the lock plate in a direction of disengaging the locking teeth from the recess and projection portion of the position keeper by receiving a pushing force from the operative surface when the sheet supplying unit is placed on the operative surface.
 3. An image forming apparatus according to claim 2, wherein: one or both of the lock plate and the operating member have an inclined surface in a contact portion where the lock plate and the operating member contact each other, the inclined surface has an inclination such that the operating member moves the lock plate to release the locking of the position keeper when the operating member receives the pushing force.
 4. An image forming apparatus according to claim 1, wherein the position keeper includes a pair of cursors arranged in a sheet widthwise direction and operatively connected with each other, and the pair of cursors are operable to hold the sheet stack on the opposite sides of the sheet stack.
 5. An image forming apparatus according to claim 4, wherein the position keeper further includes: a rack and pinion mechanism which includes two racks extending from respective bottoms of the pair of cursors and formed with driving teeth in their respective inner side surfaces, and a pinion engaged with the respective driving teeth of the two racks; and a recess and projection portion formed in an outer side surface of at least one of the racks, and having a plurality of recesses and projections alternately formed at a predetermined pitch in a moving direction thereof, the lock mechanism includes a lock plate having a locking teeth engageable with the recess and projection portion of the position keeper to keep the position keeper in the keeping position.
 6. An image forming apparatus according to claim 5, wherein: the projections of the recess and projection portion each have a round shape, and the locking teeth are saw teeth each having a sharp leading edge. 